[bhaak]

It looks like the downloads are grouped by top level group.

So you can go to https://archive.org/download/gna-rec and see all archived groups that are in the rec hierarchy.

[Steuard]

Right: As I said earlier, the list in this archive includes rec.humor.funny but not rec.arts.books.tolkien. (And I seem to recall that the alt.* hierarchy was a little less broadly propagated, which might possibly be related to why it wasn't included here.)

[astrodust]

It looks like alt is missing, sadly.

[bhaak]

There are also groups within an existing hierarchy missing. For example. rec.games.roguelike.development is there but rec.games.roguelike.nethack is missing.